If you want to make your own the program is pretty flexible but the one main requirement is making sure your animation contains 105 frames. You can of course use duplicates if you can't come up with that many unique frames of animation.

The only tool I know so far that can do it is TuneUp Utilities 2010 (which btw, is also a nice tool to keep your Windows 7 stable and cleaned).

It's possible that other tools will start to emerge. However the official Microsoft positionwas stated long ago (bottom of the page). So any solution will only be presented in the shape of third-party tools and most likely Microsoft will never provide a direct way for you to do it.

Ok, here is what I found out. Go to "C:\Windows\System32\en-US" and find the file "winload.exe.mui". If you can find another boot screen that you like, I assume that you can just name it "winload.exe.mui" and replace the original one. I believe this will work, but I recommend making a backup of your computer before trying this.

Disclaimer:
I am in no way responsible if this directly or indirectly ruins, crashes, or messes up your computer, or if you lose or make corrupt any files while trying this operation. If you do not agree with this then do not try this. Thank you for your cooperation.